Send TCP/UDP packets from the outside to a client behind a router

Question

I want to send packets to a client on a lan, say, the public address is: 15.15.15.15 and behind this, there are 3 machines: 192.168.0.10 192.168.0.11 192.168.0.12

How would format my packet so that, if I were to send it to this IP address, it'll be received, well, atleast redirected to say, 192.168.0.10.

I know that there's 192.168.0.10, since it's one of my machines and I was wondering if I can build a notification system for something using no sockets whatsoever, but just a client listening for these. (Using scapy to parse packets)

So, for example: I send a packet from outside saying "Alert!" to 15.15.15.15 and 192.168.0.12 picks it up. Wrote this just to provide some insight!

Made this image to portray the process:

Cheers.

Answer 1

You can also implement a UDP hole punching, but you need a public server with real IP address to do that.